USB and FireWire have both been a huge boon to external storage, but their performance compared to desktop drives has always lagged behind. With the development of the new Serial ATA standards, a new external storage format, external Serial ATA, is now starting to enter into the marketplace. This article will look into the new interface, how it compares to the existing formats as well as what it can mean in terms of external storage.
USB and FireWire
Before looking at the external Serial ATA or eSATA interface, it is important to look at the USB and FireWire interfaces. Both of these interfaces were designed as high speed serial interfaces between the computer system and external peripherals. USB is more general and used for a wider range of peripherals such as keyboards, mice, scanners and printers while FireWire is almost exclusively used as an external storage interface.
Even though these interfaces are used for external storage, the actual drives used in these devices are still using the older IDE drive interfaces. What this means is the external enclosure that houses the hard or optical drive has a bridge that converts signals from the USB or FireWire interface into the IDE interface used by the drive. This translation causes some degradation in the overall performance of the drive.
One of the big advantages that both of these interfaces implemented was the hot swappable ability. Previous generations of storage interfaces typically did not support the ability to have drives dynamically added or removed from a system. This feature alone is what made the external storage market explode over the last few years.
Another interesting feature that can be found with eSATA is the port multiplier. This allows a single eSATA connector to be used to connect an external eSATA chassis that provides multiple drives in an array. This can provide expandable storage in a single chassis and the ability to develop redundant storage via a RAID array.
eSATA vs. SATA
External Serial ATA is actually a subset of the additional specifications for the Serial ATA interface standard. It is not a required function, but an extension that can be added into both controller and devices. In order for eSATA to properly function both must support the necessary SATA features. This is particularly important as many early generation SATA controllers and drives do not support the Hot Plug capability that is critical for the function of the external interface.
Even though eSATA is part of the SATA interface specifications, it uses a very different physical connector from the internal SATA connectors. The reason for this is to better shield the high speed serial lines used to transfer the signals from EMI protection. It also provides a 2m overall cable length compared to the 1m for internal cables. As a result the, the two cable types can not be used interchangeably.
One of the key advantages that eSATA offers over USB and FireWire is speed. While the other two have overhead from converting the signal between the external interface and the internal IDE based drives, SATA does not have this problem. Because SATA is the standard interface used on many new hard drives, a simple converter between the internal and external connectors is required in the housing. This means that the external device should run at the same speed as an Internal SATA drive.
So, here are the speeds for the various interfaces:
- USB 1.1 15 Mbps
- FireWire (1394a) 400 Mbps
- USB 2.0 480 Mbps
- FireWire 800 (1394b) 800 Mpbs
- SATA 1.5 1.5 Gbps
- SATA 3.0 3.0 Gbps
Based on these theoretical numbers, SATA is over twice the speed of the fastest provided by Firewire and four times faster than the best USB can provide. If the eSATA devices is using the faster 3Gbps interface, there is even greater speed potential.
Most computer motherboards and system at this point do not come equipped with eSATA connectors. As a result, it is necessary to install either a PCI or PC card adapter that features the connector or use a PCI bracket that converts an internal SATA connector into an eSATA connector at the back of the machine. There is also a very limited number of drive enclosures available currently that feature the eSATA connectors. This will likely change over time.
It is still very early on in the development of eSATA controllers and devices. This will likely change with the growing number of drives being developed to be native to SATA. Much of the earliest use of the external storage devices might come from consumer electronic devices such as external storage for video game consoles of PVR devices. The possibilities provided by eSATA offer a greater amount of flexibility in external storage even as possible replacement to internal system hard drives.